60v really sounds like what happens when you have an open ground and a PC or other equipment with RF filters on them. They have an LC network on the hot and neutral, tied to ground at the center point ... hence 60v or half of line voltage if the ground floats. Since the box is OK I agree with the IG receptacle idea. It might be as simple as an installer who thinks "Isolated Ground" means isolated FROM ground. Believe me, I have seen it.
A wiggy will probably "ground" this so don't be surprised if the problem goes away when you use it to test this "ground to box".
If you wiggy from hot to equipment ground you might see 120v on the ground referenced to the box.
